The nature of gas turbines is such that they emit small amounts of undesirable pollutants into the surrounding atmosphere, particularly when using residual fuels having a high fuel-bound nitrogen content. These fuels are often the ones that are most readily obtainable and therefore the most economical to use. Although smoke, excess carbon monoxide and unburned hydrocarbons all constitute undesirable pollutants in the exhaust of state-of-the-art gas turbines, it is the emission of excess amounts of nitrogen oxides (NO.sub.x) which causes particular concern, owing to the adverse effects attributed to these gases. Thus, it becomes particularly desirable to provide a combustion control system which permits the use of such fuels in gas turbines with a minimum amount of undesirable exhaust emission.
It is well known that lowering the temperature of combustion will decrease the concentration of nitrogen oxides in the turbine exhaust gases. It has also been demonstrated that burning the turbine fuel with excess air, i.e. using a fuel-lean mixture in the combustion process, will accomplish such a temperature reduction. However, the leanness of the fuel-air mixture required to effect a flame temperature reduction at full turbine load will not support a satisfactory flame under low load or under start-up conditions. When the latter conditions prevail, the turbine will operate at poor combustion efficiency, or not at all, if the same fuel mixture is used as at full load. Incomplete burning of the fuel mixture will occur, resulting in the presence of excessive amounts of carbon monoxide and unburned hydrocarbons in the turbine exhaust.
Existing combustion control systems which attempt to solve the problem of NO.sub.x emission by the use of variable geometry, whereby the fuel is burned with excess air, frequently operate with a relatively large variation in the pressure drop across the combustor. This variation occurs as the load on the turbine changes and it has an adverse effect on the overall operation. Further, because the control mechanism employed is usually integral with the combustor, the overall structure is mechanically complex and thus costly to build and maintain. Another disadvantage resides in the fact that existing combustors cannot be easily retro-fitted to incorporate this type of control system.